Method of producing silicon steels with oriented grains by coiling with aluminum strip

ABSTRACT

A METHOD OF MANUFACTURING STEEL SHEETS HAVING A &#34;CUBE ON EDGE&#34; GRAIN TEXTURE AND HIGH MAGNETIC PROPERTIES IN THE ROLLING DIRECTION, WHICH COMPRISES THE STEPS OF USING AS A STARTING MATERIAL A STEEL STRIP CAPABLE OF RECEIVING SAID &#34;CUBE ON EDGE&#34; TEXTURE BUT NOT HAVING SAME, BY RESORTING TO A SUITABLE COMPOSITION AND TO A SUITABLE COLD ROLLING OF THE STEEL SHEET, FOLLOWED BY A DECARBURIZING ANNEALING STEP TO REDUCE THE CARBON CONTENT TO LESS THAN ABOUT 0.005%, WHEREIN ALUMINUM IS DEPOSITED ON SAID STRIP AND SUBSEQUENTLY HOT DIFFUSED THROUGHOUT THE MASS OF THE STRIP TO BE TREATED, IN THE SOLID PHASE, SAID ALUMINUM DIFFUSION TAKING PLACE BEFORE PERFORMING THE HOT SECONDARY RECRYSTALLIZATION TO PRODUCE THE DESIRED &#34;CUBE ON EDGE&#34; GRAIN TEXTURE.

United States Patent 3,756,867 METHOD OF PRODUCING SILICON STEELS WITH ORIENTED GRAINS BY COILING WITH ALUMI- NUM STRIP Pierre Brissonneau and Georges Couderchon, Grenoble, France, assignors to Compagnie des Ateliers et Forges de la Loire (St. Chamond, Firminy, St. Etiene, Jacob Holtzer), Paris, France No Drawing. Filed Oct. 13, 1970, Ser. No. 80,411 Claims priority, application France, Nov. 3, 1969, 6937712 Int. Cl. H011? 1/04 US. Cl. 148-111 5 Claims ABSTRACT OF THE DISCLOSURE A method of manufacturing steel sheets having a cube on edge grain texture and high magnetic properties in the rolling direction, which comprises the steps of using as a starting material a steel strip capable of receiving said cube on edge texture but not having same, by resorting to a suitable composition and to a suitable cold rolling of the steel sheet, followed by a decarburizing annealing step to reduce the carbon content to less than about 0.005%, wherein aluminum is deposited on said strip and subsequently hot diffused throughout the mass of the strip to be treated, in the solid phase, said aluminum diffusion taking place before performing the hot secondary recrystallization to produce the desired cube on edge grain texture.

The present invention relates to the manufacture of steel sheets of a Goss texture, or cube on edge texture, as defined by the Millers indicia (110) [001], of the higher grade.

It is known to produce silicon steel sheets with oriented grains, with the so-called cube on edge texture, with 2.8% to 3.5 Si contents.

However, when produced according to hitherto known methods these steels displayed as magnetic properties, in the case of 0.28 mm. sheets for example, watt losses constantly in excess of .80 watt, per kilogram with a 1.5-Tesla induction and a 50-Hertz frequency, in the sheet rolling direction, these losses increasing on the other hand with the sheet thickness.

This grade is now currently standardized under the symbol M4 and guaranteed for a maximum watt loss of .89 w./ kg. if the thickness is .011", under 1.5 Tesla and 50 Hertz.

Other standardized grades are already known, which the present invention is also applicable, such as MS (equivalent to M4 but having a thickness of 0.30 and 0.35 mm.), M6 (thickuess=0.35 mm.), etc.

It is the essential object of this invention to permit a gain of 0.10 to 0.20 w./kg. with respect to the abovegiven values, and therefore to obtain values of less than 0.80 w./kg. and preferably less than 0.75 w./kg. in the rolling direction in the case of a 0.28-mm. sheet, under a 1.5-Tesla induction and a SO-Hertz frequency.

The solution adopted for achieving this object consists in increasing the metal resistivity. In this direction it is known that if an element such as aluminum or additional silicon is added to a steel already containing from 2.8% to 3.5% Si the resistivity of the metal will increase considerably.

However, adding this aluminum or complementary silicon to molten metal cannot be contemplated for various reasons inherent to each element, which are set forth hereinafter:

(a) If aluminum is added to molten metal already containing about 3% of silicon, it is definitely precluded to subsequently obtain a satisfactory cube on edge texture after rolling and adequate heat treatments;

(b) If additional silicon is mixed with the molten metal already containing about 3% Si, in order to obtain a higher than 3.5% content and increase the resistivity value, this metal cannot be cold rolled on a commercial basis.

To avoid these difficulties, the aluminum must be added by diffusing same into the metal in the solid phase.

To this end, the first object of the present invention consists in providing a method of producing steel sheets with grains oriented according to the so-called cube on edge [001] texture having high magnetic properties in the rolling direction, this method consisting in using as an initial material a steel strip capable of receiving the cube on edge grain texture (but not having same so far), by selecting a suitable steel composition and a suitable cold rolling operation followed by a decarburizing annealing step for reducing the carbon content to less than 0.005% this method being characterized in that aluminum is deposited on said strip and subsequently diffused in the hot state throughout the metal mass to be treated, in the solid phase, this aluminum diffusion being performed before carrying out in the hot state the secondary recrystallization to provide the desired cube on edge grain texture.

It is another object of this invention to provide a method of manufacturing steel sheets with grains oriented according to the so-called cube on edge texture, which are characterized by watt loss, characteristics in the rolling direction, of less than 0.80 watt per kilogram, and even less than 0.75 w./kg., in the case of a sheet having a thickness of 0.28 mm., under a 1.5-Tesla induction and a SO-Hertz frequency, this method consisting in utilizing as a starting material a steel produced according to a known technical process, which contains from 2.8% to 3.5% of silicon, and capable, after suitable rolling and heat treatments, of receiving a cube on edge grain texture, casting said steel into a continuous-casting ingots or slabs, rolling the steel ingots or slabs thus obtained into strips having a thickness within the range of from 1.8 mm. to 2.6 mm., and preferably of about 2 mm., removing the oxide layer, cold-rolling the metal to a thickness of less than 0.40 mm. and preferably of about 0.28 mm. through one or several steps separated by suitable heat treatments, applying a decarburizing annealing step under wet hydrogen conditions to the metal in order to obtain a cold rolled strip having a carbon content inferior to 0.005%, this method being characterized in that the thus rolled strip decarburized to less than 0.005 C is coiled in conjunction with an aluminum strip of a thickness suflicient to provide at least 0.7% Al and preferably from 1% to 2% Al in the metal after subsequent diffusion, said coiling step being followed by two successive heat treatments at two different temperatures which are carried out in the same or in two different annealing furnaces, with or without return to room temperature between the two treatments, the first treatment carried out at a lower temperature ensuring the desired diffusion of aluminum within the metal strip to be treated, the second treatment, carried out at a higher temperature, ensuring the secondary recrystallization to provide the desired cube on edge grain texture.

According to another feature characterizing this invention, the thickness of the aluminum strip is selected as a function of the desired average aluminum content in the metal after the diffusion step.

According to a further feature characterizing this invention, the thickness of the aluminum strip is of the order of microns if it is desired to obtain about 1.1% of aluminum after diffusion in a sheet having a final thickness of 0.28 mm.

It is a complementary feature of this invention that the width of the aluminum strip is at least equal to that of the strip to be treated.

According to another specific feature of the present invention, the first heat treatment is performed at a temperature within the range of from 660 C. to 950 C., and preferably from 660 C. to 800 C., and the second heat treatment takes place at a temperature within the range of from 950 C. to l,250 C.

Moreover, a feature characterizing this invention is that the time required for each heat treatment varies in the opposite direction to the treatment temperature.

According to a specific feature characterizing this invention, each one of the two successive heat treatments is characterized by the fact that the product is kept at a constant temperature or at a temperature varying slightly during a time period of at least one hour, and preferably of 6 to 48 hours.

It is an additional feature of this invention that the whole or part of the aluminum can be replaced by silicon, germanium or cobalt.

This invention is also concerned with steel sheets obtained by applying the method set forth hereinabove and having definitely reduced watt losses characteristics in the rolling direction, for example less than 0.80 w./kg., and even less than 0.75 w./kg., for a thickness of 0.28 mm. under a 1.5-Tesla induction and a SO-Hertz frequency.

As will be readily understood by those conversant with the art, it is an advantageous feature of this invention that, while producing a metal having a good cube on edge grain texture identical with that already obtained with known processes, the metal resistivity is improved through commercially applicable means by depositing and diffusing aluminum in the solid phase (in contrast to the deposit and diffusion of silicon in solid phase which are extremely difiicult and costly to carry out in actual practice) while reducing the anisotropy constant K1, thus permitting of reducing the detrimental effects, from the magnetic point of view, of the slight uncertainties or confusions observed in all cases in the commercial manufacture of steel having a cube on edge grain texture.

It is another advantageous feature of the method of this invention that the aluminum liquefied at the beginning of the diffusion annealing step adheres to the metal to be treated without flowing, by simple capillarity, thus aifording a homogeneous distribution of said aluminum throughout the steel mass at the end of the diffusion step.

Furthermore, this invention is advantageous in that the liquid aluminum reduces the silica and the oxides of metals less electro-positive than aluminum, which oxides always remain on the skin of the metal after the decarburizing treatments and develop alumina acting as an anti-adhesive layer between the successive turns of the coil during the secondary recrystallization annealing step.

It is a complementary advantage of this invention that aluminum due to its behaviour when associated with iron penetrates into the metal sheet without causing inclusions to develop near the surface and exert a detrimental effect on the magnetic properties of the product, notably in connection with watt losses.

In order to afford a clearer understanding of this invention, a typical method of manufacturing steel sheets according to this invention, which have watt losses of about 0.75 w./k-g. in the rolling direction for a thickness of 0.28 mm. (0.011") under 1.5 Tesla and 50 Hertz, will now be described by way of example but not of limitation.

A very low carbon steel containing 0.030% carbon maximum is processed in a open hearth furnace.

By applying known techniques ferro-silicon is added to this molten steel in order to increase the silicon content to 3.25%, and the resulting metal is cast into 7-ton ingots, after adding to the melt metal at least one of the elements capable of subsequently promoting the development of the cube on edge texture.

Then the steel is hot rolled until a 2-mm. thick coiled strip is obtained. This coil is then pickled in a 15% sulfuric acid solution at a temperature of 60 to C.

The pickled coil is then cold rolled a first time to a thickness of 0.58 mm., then shortly heat treated (during afew seconds) at 925 C. in a dry hydrogen atmosphere.

The coil is then cold rolled again to a thickness of 0.28 mm. and subsequently annealed for decarburization in a wet hydrogen atmosphere at 815 C. during 2 minutes.

Then the coil is uncoiled and recoiled in sandwich form with a strip of pure aluminum having a thickness of 10 microns.

The coil thus obtained is box annealed at 725 C. during 24 hours in a dry hydrogen atmosphere, thus causing the aluminum to diffuse in the strip to be treated up to an Al content of 1.1%, whereafter a second box annealing step is performed at 1,180 C. during 24 hours, this second annealing step producing the secondary recrystallization giving the desired cube on edge grain texture. The resistivity of the metal thus obtained is 62 microohms/sq. cm./cm., the resistivity of a similar metal but without the aluminum diffusion being about 50 microohms/sq. cm./cm.

The product thus obtained is characterized by a watt loss of 0.75 w./ kg. in the rolling direction under 1.5 Tesla and 50 Hertz.

Of course, it would not constitute a departure from the present invention to bring various modifications and improvements in details thereof, and also to use means equivalent to those described hereinabove.

More particularly, this invention is applicable to thin sheets of different thicknesses, preferably less than 0.40 mm.

What is claimed is:

1. A method of manufacturing a steel sheet having a cube-on-edge grain texture and high magnetic properties in the rolling direction thereof, which comprises casting steel containing 2.8-3.5% silicon, rolling the cast steel into a strip, removing oxides from the strip, cold-rolling the strip to a thickness of less than 0.40 mm., subjecting the cold-rolled strip to a decarburization and annealing to reduce the carbon content of the strip to less than 0.005%, coiling the decarburized strip in conjunction with an aluminum strip of a thickness sufficient to provide an aluminum content of at least 0.7% in the strip after diffusion of the aluminum into the strip, annealing the coiled strip having the aluminum deposited thereon at 660-950 C. for 6-48 hours in a dry hydrogen atmosphere to diffuse the aluminum into the strip, and annealing the resultant strip at 950-1250 C. for 6-48 hours to cause secondary recrystallization, thus providing the cube-on-edge grain texture.

2. A method as set forth in claim 1, wherein the thickness of the aluminum strip is selected as a function of the average aluminum content to be obtained in the metal after the diffusion step.

3. A method, according to claim 1, of manufacturing a steel sheet having a final thickness of 0.28 mm. wherein the thickness of the aluminum strip is of about 10 microns in order to obtain after the diffusion about 1.1% of aluminum in said sheet.

4. A method as set forth in claim 1, wherein the width of the aluminum strip is at least equal to that of the strip to be treated.

5. A method according to claim 1, wherein the first heat treatment for diffusing the aluminum in the metal to be treated is carried out at a temperature within the range of 660 C. to 800 C.

(References on following page) 6 References Cited FOREIGN PATENTS UNITED STATES PATENTS 1,806,863 9/1963 Japan 148--100 1,836,862 11/1962 Japan 148-3155 12/1965 SIXWS et 148-413 1,210,022 2/1966 Germany 14s -31.ss 11/1961 Moblus 148--120X 45 1/ 1969' Ames et 143113 X L. DEWAYNE RU'I'LEDGE, Primary Examiner fixi g- 3 233 W. R. SATTERFIELD, Assistant Examiner 9/1963 Albert 148-120 US. Cl. X.R.

12/1929 McKeehan 148-122X 50 14 2, 113 

